Coupling of acoustic window and lens for medical ultrasound transducers

ABSTRACT

A method and structure for improving the coupling of an acoustic window or lens to a RFI shield by modifying the surface of the shield to promote adhesion. The surface of the shield can be chemically modified with an epoxy and/or mechanically modified by creating an unsmooth top surface.

FIELD OF THE INVENTION

This invention relates to transducers and more particularly to broadbandphased array transducers for use in the medical diagnostic field.

Ultrasound machines are often used for observing organs in the humanbody. Typically, these machines incorporate transducer arrays forconverting electrical signals into pressure waves and vice versa.Generally, the transducer array is in the form of a hand-held probewhich may be adjusted in position while contacting the body to directthe ultrasound beam to the region of interest. Transducer arrays mayhave, for example, 128 phased transducer elements for generating anultrasound beam.

An electrode is placed at the front and rear portion of the transducerelements for individually exciting each element. The pressure wavesgenerated by the transducer elements are directed toward the object tobe observed, such as the heart of a patient being examined. Each timethe pressure wave confronts tissue having different acousticcharacteristics, a portion of the ultrasound wave is reflected backward.The array of transducers may then convert the reflected pressure wavesinto corresponding electrical signals. An example of a phased arrayacoustic imaging system is described in U.S. Pat. No. 4,550,607 grantedNov. 5, 1985 to Maslak et al. and is incorporated herein by reference.That patent illustrates circuitry for focusing the incoming signalsreceived by the transducer array in order to produce an image on thedisplay screen.

The elevation focus of most phased array transducers can generally becategorized as lens focused or mechanically focused. In the case of lensfocused transducer arrays the emitting surface of the array is flat inthe elevation direction and a material, the lens, is placed between theobject to be imaged and the array. The lens material has a lowervelocity of sound than the object being imaged for a convex shaped lenssurface. The focusing of the ultrasound beam is achieved through therefraction at the lens/object interface. U.S. Pat. Nos. 4,686,408 and5,163,436 describe lens focused phased array transducers and arespecifically incorporated herein by reference.

Mechanically focused transducer arrays utilize a piezoelectric layer andmatching layers which have a curved surface which face the object to beimaged. The surface is curved along the elevation direction and formseither a concave or convex structure. U.S. Pat. Nos. 4,184,094 and4,205,686 described such a mechanically focused transducer array and arehereby specifically incorporated by reference. The curved surface of thetop matching layer is then covered by an acoustic window which isusually formed from a low attenuation polymeric material. The polymericmaterial is considered an acoustic window and provides no focusing ofthe acoustic beam. Several two part castable polyurethane materials canbe used as acoustic windows.

With reference to acoustic windows, it is possible to use polymericmaterials with acoustic attenuation as low as 1.0dB/mm at around 7 MHzand sound velocity very close to that of human body tissue, i.e. about1,540 m/s. The use of low loss acoustic window materials in combinationwith a mechanically focused stack results in better depth penetration ofthe acoustic beam into the human body being imaged. With reference tothe lens structure previously described, the lens material is much moreattenuative than the acoustic window material, i.e. around 5.5 dB/mm atabout 7 MHz, with velocity of sound far below that of water or the humanbody. Lower velocity materials such as these provide focusing of theacoustic beam in the elevation plane. Typically silicone RTV materialsare used as these later lens materials.

Often a radio frequency interference (RFI) shield is provided underneaththe lens or window. The RFI shield is used to reduce electromagneticinterference caused by the hospital or clinical environment which wouldproduce noise in the ultrasound image or vice versa. There are varioustechniques for employing an RFI shield. One form of shield includes apolymeric shield substructure that has been sputtered with a thin metal.Often, the metal of choice is gold because gold has good conductivityand relative stability in the presence of various deleteriousdisinfecting solutions used by hospitals and clinics to disinfect thetransducer between patient use.

A disadvantage associated with polymeric materials used to form windowsor lenses is that they all absorb liquids to some extent, polyurethanesmore so than silicones. The gold of the RFI shield acts as a relativelyinert barrier to further ingress of these solutions that might occur asa result of repeated soaking of the transducer between patient uses toachieve a satisfactory level of disinfection or sterilization.

It is often difficult to achieve adequate adhesion between the window orlens material and the typically smooth metal surface of the RFI shield.In addition, it is often difficult to maintain adequate adhesionthroughout the lifetime of the probe due to the repeated soakings of thetransducer probe in disinfecting solution between patients. Transducertypically may be subjected to over 1,000 disinfecting cycles in a year,each cycle lasting anywhere from about 20 to 45 minutes. If thetransducer probe is subject to sterilization, the soak time may be aslong as 10 hours. The eventual ingress and diffusion of the disinfectingsolutions through the window or lens of the transducer causesdelamination of the window or lens from the underlying RFI shield.Delamination of this type results in poor image quality and typicallythe transducer must be returned to the manufacturer.

It is thus desirable to provide a method for optimizing the adhesion ofthe window or lens material to the underlying RFI shield so thatrepeated disinfection or sterilization cycles do not compromise theintegrity of the transducer structure.

It is also desirable to promote the adhesion between the window or lensmaterial with the underlying shield in a simple and inexpensive manner.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided ashield for reducing the impact of radio frequency interference for anultrasound transducer having a shield substructure including a polymericfilm, a layer of metal disposed on the polymeric film and a layer ofepoxy material coating the layer of metal.

According to a second aspect of the present invention there is provideda method of manufacturing a shield for reducing the influence of radiofrequency interference for an ultrasound transducer. The method includesproviding an acoustic stack, providing a shield substructure over aportion of the acoustic stack so that a top surface of the shieldsubstructure faces an object to be imaged and treating the top surfaceof the shield substructure to promote adhesion with a covering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art transducer array 10 for generating anultrasound beam.

FIG. 2 illustrates a cross-sectional view of a transducer arrayaccording to a first preferred embodiment of the present invention.

FIG. 3 illustrates a portion of the RFI shield shown in FIG. 2 takenalong the elevation direction.

FIG. 4 illustrates a cross-sectional view of a transducer arrayaccording to a second preferred embodiment of the present invention.

FIG. 5 illustrates a cross-sectional view of a transducer arrayaccording to a third preferred embodiment of the present invention. Thecross-section is taken along the elevation direction.

FIG. 6 illustrates a cross-sectional view of a transducer arrayaccording to a fourth preferred embodiment of the present invention.

FIG. 7 illustrates a cross-sectional view of a transducer arrayaccording to a fifth preferred embodiment of the present invention.

FIG. 8 illustrates a cross-sectional view of a transducer arrayaccording to a sixth preferred embodiment of the present invention.

FIG. 9 illustrates a cross-sectional view of a transducer arrayaccording to a seventh preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 illustrates generally a transducer array 10 for generating anultrasound beam. Typically, such an array may have 128 transducerelements 12 in the azimuthal direction. Adapted from radar terminology,the x, y, and z directions are referred to as the azimuthal, elevation,and range directions, respectively.

Each transducer element 12, typically rectangular in cross-section, maycomprise a first electrode 14, a second electrode 16 and a piezoelectriclayer 18. In addition, one or more acoustic matching layers 20 may bedisposed over the piezoelectric layer 18 to increase the efficiency ofthe sound energy transfer to the external medium. The electrode 14 for agiven transducer element 12 may be part of a flexible circuit 15 forproviding the hot wire or excitation signal to the piezoelectric layer18. Electrode 16 for a given transducer element may be connected to aground shield return 17. To further increase performance, thepiezoelectric layer 18 may be plated or metalized on its top and bottomsurfaces and the matching layer 20 may also be plated or metalized onall surfaces so that electrode 16 which is in physical contact with thematching layer 20 is electrically coupled to a surface of thepiezoelectric layer 18 by the plating.

The transducer elements 12 are disposed on a support or backing block24. The backing block 24 may be highly attenuative such that ultrasoundenergy radiated in its direction (i.e., away from an object 32 ofinterest) is substantially absorbed. In addition, a mechanical lens 26may be placed on the matching layer 20 to help confine the generatedbeam in the elevation-range plane and focus the ultrasound energy to aclinically useful depth in the body. Alternately the piezoelectric layer18 may have a curved surface facing the object to be imaged and a lowloss acoustic window may be disposed over the piezoelectric layer suchas the transducer array described in U.S. Pat. No. 5,415,175 granted May16, 1995 to Hanafy et al. which is specifically incorporated herein byreference.

Alternatively, the piezoelectric layer 18 may have a plano-concave shapesuch as that disclosed in U.S. Pat. No. 5,415,175 to Hanafy et al.issued May 16, 1995 which is specifically incorporated herein byreference. The concave surface faces the object 32 to be imaged.

The transducer array 10 may be placed in a nose piece 34 which housesthe array. Examples of prior art transducer structures are disclosed inCharles S. DeSilets, Transducer Arrays Suitable for Acoustic Imaging,Ph.D. Thesis, Stanford University (1978) and Alan R. Selfridge, Designand Fabrication of Ultrasonic Transducers and Transducer Arrays, Ph.D.Thesis, Stanford University (1982).

The materials used to form the various parts of the transducer array arecommonly used in the transducer area. The backing block 24 and acousticmatching layers 20 can be manufactured using common epoxies or urethaneas can be purchased from Hysol of Pittsburgh, Calif., for example.Fillers such as aluminum oxide may also be used. The materials can beoptimized to reduce the reflection of the acoustic energy at the layerinterfaces. In a preferred embodiment, the backing block 24 is formed ofan acoustic absorbing material which absorbs spurious and unwantedacoustic energy. In a preferred embodiment, the backing block may beformed of a filled epoxy comprising Dow Corning's part number DER 332treated with Dow Corning's curing agent DEH 24 and has an aluminum oxidefiller.

Individual elements 12 can be electrically excited by electrodes 14 and16 with different amplitude and phase characteristics to steer and focusthe ultrasound beam in the azimuthal-range plane. An example of a phasedarray acoustic imaging system is described in U.S. Pat. No. 4,550,607issued Nov. 5, 1985 to Maslak et al. and is specifically incorporatedherein by reference. U.S. Pat. No. 4,550,607 illustrates circuitry forcombining the incoming signals received by the transducer array toproduce a focused image on the display screen. When an electrical signalis imposed across the piezoelectric layer 18, the thickness of the layermomentarily changes slightly. This property is used to generate soundfrom electrical energy. Conversely, electrical signals are generatedacross the electrodes in contact with the piezoelectric layer 18 inresponse to thickness changes that have been imposed mechanically.

The pressure waves generated by the transducer elements 12 are directedtoward an object 32 to be observed, such as the heart of a patient. Eachtime the pressure wave confronts tissue having different acousticcharacteristics, a portion of the wave is reflected backward. The arrayof transducers may then convert the reflected pressure waves intocorresponding electrical signals.

For the transducer shown in FIG. 1 the beam is said to be mechanicallyfocused in the elevation direction by mechanical lens 26. The focusingof the beam in the azimuthal direction is done electronically bycontrolling the timing of the transmissions and receptions of eachtransducer element. This may be accomplished by introducing appropriatephase delays in the firing and switching signals.

Reflected energy from a particular location in the image plane iscollected by the transducer elements. The resultant electronic signalsfrom individual transducer elements are individually detected andreinforced by the focusing delays. Extensive processing of such datafrom the entire image plane is done to generate an image of the object.Such an image is typically displayed on a CRT monitor at 10 to 30frames/second.

FIG. 2 illustrates a cross-sectional view of a transducer array 50according to a first preferred embodiment of the present invention. Thecross-section is taken along the elevation direction. The array 50includes an acoustic stack 52, an RFI shield 54 and a low loss acousticwindow 56. The acoustic stack 52 includes a backing block, signal andground flex circuits, piezoelectric crystal and, if desired, acousticmatching layers such as that shown in FIG. 1. The details of theacoustic stack in FIGS. 2, 4-9 are not illustrated. In a preferredembodiment a plano-concave piezoelectric layer is used, thus the topsurface of the acoustic stack 52 is illustrated as concave in shape. Anose piece 58 surrounds the acoustic stack 52, RFI shield 54 and thesides of the low loss acoustic window 56. The RFI shield 54 in thispreferred embodiment includes a polymeric shield substructure having atleast one layer of refractory metal thereon. The RFI shield structurewill be described in greater detail with reference to FIG. 3. In apreferred embodiment, to promote the adhesion between the low lossacoustic window material 56 and the RFI shield 54, the top surface ofthe RFI shield 54 is modified. In this preferred embodiment the topsurface of the RFI shield 54 is treated with a primer 60, preferably aliquid organofunctional silane (or titanate or zirconate) primer such asDOW CORNING 1200 or CHEMLOK AP131 commercially available from LordCompany of Erie, Pa. Then an epoxy seed layer 62 is deposited over theprimed top surface of the shield 54. Preferably the epoxy seed layer 62is no more than about 5 microns thick. Once the epoxy seed layer 62 iscured, the low loss polyurethane window material 56 is cast directly ontop of the epoxy layer 62.

In a preferred embodiment, the window material is a low losspolyurethane such as U-2008 manufactured by Castall Inc. of WeymouthIndustrial Park, East Weymouth, Mass. The epoxy seed layer 62 ispreferably a premixed, frozen adhesive such as Hysol RE2039 commerciallyavailable from Dexter-Hysol of Los Angeles, Calif. The nose piece 58 ispreferably formed from a rigid engineering thermoplastic such as RADELmanufactured by Amoco.

In an alternate embodiment, the top surface of the RFI shield 54 may notbe primed with a liquid silane primer although this has been shown topromote the adhesion of the window material 56 to the RFI shield 54. Themetal of the RFI shield 54 may be selected from the group includinggold, titanium, chromium or alloys thereof. The polymeric shieldsubstructure may be selected from the group including polyimide,polyester, polyurethane, or PEN. The low loss window may be a polymerselected from the group consisting of MDI polyester based polyurethane,MDI polyether based polyurethane, TDI polyester based polyurethane, TDIpolyether based polyurethane, polybutadiene and polyether/polyesterpolyurethane copolymer.

The reliability of the resulting bond between the window material 56 andthe RFI shield 54 has been demonstrated using standard lap shearcoupons. The lap shear coupons simulate the structure shown in FIG. 2and were shown to withstand a 30 day soak in the following commerciallyavailable liquid disinfectants such as CIDEX, GIGASEPT ff, ENDOSPOR,MATAR and VIRKON and still retained a bond strength of no less than 60%of the strength measured on unsoaked control coupons of a similarstructure.

FIG. 3 illustrates a portion of the RFI shield 54 shown in FIG. 2 takenalong the elevation direction. In a preferred embodiment, the shield 54includes a shield substructure 64, a seed layer 66 on both sides of theshield substructure 64 and a layer of metal 68 over the seed layers 66.In a preferred embodiment the shield substructure 64 may be formed ofpolyimide, the adhesion layer 66 is preferably formed of titanium andthe metal 68 is gold. The shield substructure 64 is first coated withthe titanium seed layer 66 and then the metal layer 68 is depositedthereon preferably by sputtering. The titanium seed layers 66 underneaththe layer of refractory metal help promote the integrity of the shield.

FIG. 4 illustrates a cross-sectional view of a transducer array 70according to a second preferred embodiment of the present invention. Thecross-section is taken along the elevation direction. The transducerarrays in the following figures are similar to that already described inFIG. 2 and thus the same reference numerals will be used to describesimilar elements. This preferred embodiment is similar to that shown inFIG. 2 except that the layer of refractory metal on the top of the RFIshield 54 has been removed and the low loss polyurethane window material56 is directly bonded to the polymeric shield substructure 64. The topsurface of the shield substructure 64 may be treated with an epoxy. Theresulting bond strength between the window material 56 and the polymericshield structure 64 is significantly stronger than the bond between thewindow material and the smooth surface of a refractory metal. The layerof metal needed for the RFI shield 54 is located on the underside of thepolymer shield substructure 64 as illustrated.

With reference to FIGS. 2-4, the top surface of the RFI shield ismodified chemically. It is also possible to mechanically modify the topsurface of the RFI shield to promote adhesion between the windowmaterial and the RFI shield. FIGS. 5 and 6 illustrate two alternateembodiments for mechanically modifying the top surface of the RFIshield.

FIG. 5 illustrates a cross-sectional view of a transducer array 80according to a third preferred embodiment of the present invention. Thecross-section is taken along the elevation direction. In this preferredembodiment, the top surface 82 of the layer of refractory metal 68' isnot smooth. The unsmooth top surface allows for mechanical bondingbetween the window material 56 and the refractory metal 68'. In additionto mechanical modification, the top surface of the refractory metal maybe chemically modified as well as shown in FIG. 2 to provide chemicalbonding as well as mechanical bonding. Various techniques may be used toprovide an unsmooth top surface of the refractory metal including plasmaetching the shield substructure after it has been metalized oralternatively, before the shield substructure is metalized, using arough polymeric shield material such as polyimide so that the layer ofrefractory metal will conform to this unsmooth surface. Alternatively,holes, slots or patterns may be formed in the polymeric shield materialto create an unsmooth surface.

FIG. 6 illustrates a cross-sectional view of a transducer array 90according to a fourth preferred embodiment of the present invention. Thecross-section is taken along the elevation direction. In this preferredembodiment either a window material 56 or lens material 102 may be caston the RFI shield. In this preferred embodiment, a plurality of holes 92have been drilled in the top surface of the RFI shield 54'. The windowmaterial or lens material can then be cast on the RFI shield. The windowor lens material fills these holes which serve as anchors tomechanically enhance the adhesion between the window or lens materialand the metal of the RFI shield. The holes may be drilled with a laseror they may be precisely located using standard photolithography and dryor wet etching techniques. The size of the holes and their spacing mustbe small compared to the acoustic wavelength in the transducer,preferably in the submicron range.

The embodiments shown in FIGS. 5 and 6 thus provide a mechanical bondbetween the window or lens material and the RFI shield. The mechanicalbond may be enhanced by also providing a chemical bond as shown in FIGS.2 and 4.

FIG. 7 illustrates a cross-sectional view of a transducer arrayaccording 100 to a fifth preferred embodiment of the present invention.The cross-section is taken along the elevation direction. In thispreferred embodiment a RTV silicone focusing lens 102 is used inconjunction with a titanium 66 coated polymeric shield substructure 64has been primed with a liquid organofunctional silane primer 60. Thethickness of the titanium 66 can be as thin as a few hundred Angstroms.In an alternate embodiment, the shield substructure 64 may be providedwith a layer of refractory metal which has been plasma etched or UVOzone cleaned prior to casting the RTV silicone to optimize the adhesionbetween the RFI shield and the lens material. In a preferred embodiment,the polymeric shield substructure is formed of polyimide or polyesterwhich has been first primed with a liquid organofunctional silane primersuch as DOW CORNING 1200.

The bond strength resulting from the selection of materials shown inFIG. 7 has been analytically tested. The resulting bond strength of anRTV silicone such as DOW CORNING Q8008 to a titanium coated shield whichhas been primed with DOW CORNING liquid silane primer 1200 is able towithstand a prolonged soak for 30 days in a variety of liquiddisinfectants. This bond strength was measured by assembling lap shearcoupons similar to those previously described. For the testing, the RTVsilicone material is bonded between two rigid coupons. The lower couponis a solid aluminum coupon that has been sputtered with titanium tosimulate the layer of refractory metal of the RFI shield. The uppercoupon is a stainless steel coupon with a mesh pattern that allowspenetration of the liquid disinfectant in which the coupon is soaked toingress into the silicone RTV. The lower coupon's titanium sputteredsurface is cleaned in a manner that would be used in the manufacturingprocess to clean a typical RFI shield, i.e., mild aqueous detergentclean followed by a deionized water rinse and low temperature over dry.Following this cleaning the titanium is primed preferably using DOWCORNING 1200 liquid silane primer similar to the process used on anactual RFI shield top surface metalization. Coupons prepared in thismanner have been soaked in various liquid disinfectants typically usedin hospitals and clinics for up to 30 days and then pulled apart usingan Instron tensile instrument. Adhesive bond strengths between the RTVsilicone lens material and the titanium on the lower coupons have beenshown to exceed 400 psi on those coupons which have been soaked for 30days in the following commercially available disinfectants: CIDEX,ENGARDE, GIGASEPT, GIGASEPT ff, KORSOLEX, SEKUSEPT EXTRA, and VIRKON.

FIG. 8 illustrates a cross-sectional view of a transducer array 200according to a sixth preferred embodiment of the present invention. Thecross-section is taken along the elevation direction. In this preferredembodiment, a low loss polymeric material 56 such as polybutadiene iscompression molded directly onto the housing. In a preferred embodimentthe housing is made of a rigid high temperature engineeringthermoplastic such as RADEL manufactured by Amoco. The acoustic stack 52is inserted into the nose piece 58 after the polymeric window 56 hasbeen compression molded onto the patient facing end of the transducer.The RFI shield 54 may be prepared in any one of the numerous waysalready discussed to optimize the adhesion between the RFI shield andthe polymeric window. For example, the layer of refractory metal of theRFI shield could be plasma etched, UV Ozone cleaned or primed with aliquid silane primer prior to coating with an epoxy adhesive such asHysol's RE2039.

The use of a compression molded polymer such as polybutadiene providesseveral advantages. The class of rubbers which can be compression moldedare very tough, i.e. they have a high tear strength, and are chemicallyresistant to prolonged soaks in typical liquid disinfectants. Inaddition, the process of compression molding directly onto the nosepiece results in an environmental seal around the entire nose pieceperimeter. Such a seal deters the penetration of disinfectants andcoupling gels around the sides of the polymeric window which mighteventually compromise the adhesion of the window to the underlying RFIshield.

The compression molded polymer described above could also bemanufactured as a premolded structure. This premolded structure could beattached to the RFI shield or acoustic stack using adhesives such asHysol's RE2039. Details could be manufactured on the inside of the nosepiece housing to capture this premolded structure so that its locationis consistently assured.

FIG. 9 illustrates a cross-sectional view of a transducer array 300according to a seventh preferred embodiment of the present invention.The cross-section is taken along the elevation direction. Liquiddisinfectants may also seep into the transducer between the nose pieceand acoustic stack. In a preferred embodiment a chemical barrier in theform of an RTV silicone bead 104 is located between the nose piece 58and the acoustic stack 52. The bead 104 encircles the perimeter of theacoustic stack 52. The bead minimizes the ingress of disinfectants orother chemicals used to clear or image ultrasound transducers such ascoupling gels around the edges of the acoustic stack. The bead 104 mayalso be used in conjunction with the previously described embodiments.

It is to be understood that the forms of the invention describedherewith are to be taken as preferred examples and that various changesin the shape, size and arrangement of parts may be resorted to, withoutdeparting from the spirit of the invention or scope of the claims.

What is claimed is:
 1. A shield for use in a transducer, the shield disposed between an acoustic stack located in the interior of the transducer and capable of transmitting and receiving ultrasound signals upon excitation and a barrier element shielding the interior of the transducer from an environment external to the transducer, the shield comprising:a shield substructure having a top surface and a bottom surface wherein the top surface faces the barrier element and the bottom surface faces the acoustic stack; and a layer of refractory metal disposed on the top surface of the shield substructure wherein the layer of refractory metal promotes the adhesion of the barrier element to the top surface of the shield substructure wherein the metal is selected from the group consisting of titanium, titanium alloys, chromium and chromium alloys.
 2. A shield according to claim 1 wherein the shield substructure is a polymer selected from the group consisting of polyimide, polyester, polyurethane and polyethylenaphthalate.
 3. A shield according to claim 1 wherein the barrier element is an acoustic window.
 4. A shield according to claim 3 wherein the acoustic window is a polymer selected from the group consisting of diphenyl-methane-diisocyanate polyester based polyurethane, diphenyl-methane-diisocyanate polyether based polyurethane, toluene diisocyanate polyester based polyurethane, toluene diisocyanate polyether based polyurethane, polybutadiene and polyurethane copolymer.
 5. A shield according to claim 1 wherein the barrier element is an acoustic lens.
 6. A shield according to claim 5 wherein the acoustic lens is formed of silicone room temperature vulcanized.
 7. A shield according to claim 1 further comprising a layer of primer disposed between the layer of metal and the barrier element.
 8. A shield according to claim 7 wherein the primer is selected from the group consisting of liquid organofunctional silane, liquid organofunctional titanate and liquid organofunctional ziroconate.
 9. A shield according to claim 1 further comprising a layer of epoxy disposed between the layer of metal and the barrier element.
 10. A shield according to claim 1 wherein the layer of refractory metal is textured so that it has an uneven surface.
 11. A shield according to claim 10 wherein the layer of refractory metal has a plurality of indentations formed therein.
 12. A shield according to claim 1 wherein the top surface of the shield substructure is textured so that it has an uneven surface.
 13. A shield according to claim 1 further comprising an impermeable bead of material located in the interior of the transducer and encircling the perimeter of the acoustic stack to reduce the ingress of material in the environment external to the transducer from entering the interior of the transducer.
 14. A shield for use in a transducer, the shield disposed between an acoustic stack located in the interior of the transducer and capable of transmitting and receiving ultrasound signals upon excitation and a barrier element shielding the interior of the transducer from an environment external to the transducer, the shield comprising:a shield substructure having a top surface and a bottom surface wherein the top surface faces the barrier element and the bottom surface faces the acoustic stack, wherein the top surface of the shield substructure is textured to promote the adhesion of the barrier element to the too surface of the shield substructure, wherein the top surface of the shield has a plurality of indentations formed therein.
 15. A shield according to claim 14 wherein the plurality of indentations form a pattern.
 16. A shield according to claim 14 wherein the plurality of indentations have substantially the same shape.
 17. A shield according to claim 14 wherein the shield substructure is a polymer selected from the group consisting of polyimide, polyester, polyurethane and polyethylenaphthalate.
 18. A shield according to claim 14 wherein the barrier element is an acoustic window.
 19. A shield according to claim 18 wherein the acoustic window is a polymer selected from the group consisting of diphenyl-methane-diisocyanate polyester based polyurethane, diphenyl-methane-diisocyanate polyether based polyurethane, toluene diisocyanate polyester based polyurethane, toluene diisocyanate polyether based polyurethane, polybutadiene and polyether/polyester polyurethane copolymer.
 20. A shield according to claim 14 wherein the barrier element is an acoustic lens.
 21. A shield according to claim 20 wherein the acoustic lens is formed of silicone room temperature vulcanized.
 22. A shield according to claim 14 further comprising a layer of primer disposed over the top surface of the shield substructure.
 23. A shield according to claim 22 wherein the primer is selected from the group consisting of liquid organofunctional silane, liquid organofunctional titanate and liquid organofunctional ziroconate.
 24. A shield according to claim 14 further comprising a layer of refractory metal disposed over the top surface of the shield substructure wherein the layer of metal is textured so that it has an uneven surface.
 25. A shield according to claim 24 wherein the layer of metal is a refractory metal selected from the group consisting of titanium, titanium alloys, chromium and chromium alloys.
 26. A shield according to claim 24 further comprising a layer of epoxy disposed on the layer of metal.
 27. A shield according to claim 24 wherein the layer of refractory metal has a plurality of indentations formed therein.
 28. A shield according to claim 24 further comprising a layer of primer disposed on the layer of metal.
 29. A shield according to claim 14 further comprising an impermeable bead of material located in the interior of the transducer and encircling the perimeter of the acoustic stack to reduce the ingress of material in the environment external to the transducer from entering the interior of the transducer. 